Ovulation Calculator: Fertile Window & Dates
Estimate your fertile window and likely ovulation day from your last period and cycle length. An estimate for planning, not a contraception method. Nothing is stored.
Estimate your fertile window and likely ovulation day from your last period and cycle length. An estimate for planning, not a contraception method. Nothing is stored.
Your fertile window
Pick your last period date
Ovulation is estimated in the middle of that window.
These are estimates based on an average cycle. Real ovulation shifts from month to month, so this is not a reliable way to avoid pregnancy. If your cycles are very irregular, see our guides to PCOS and progesterone. This tool is information, not medical advice.
How it works
Enter the first day of your last period and your usual cycle length to estimate your fertile window and likely ovulation day.
The formula and sources
It places ovulation about 14 days before your next expected period, and marks the fertile window as the five days before ovulation through ovulation day, when conception is most likely.
Assumptions
- Your cycles are fairly regular.
- The luteal phase is close to the typical 14 days.
Limitations
This is an estimate for planning, not a contraception method. Ovulation shifts month to month, and stress, illness, and cycle changes move it.
When to see a doctor
If your cycles are very irregular, or you have been trying to conceive without success, speak to your doctor. Our guide to perimenopause covers changing cycles later in life.
Primary sources
- Frontiers in Medicine, 2018. Sets the fertile window as the day of ovulation and the five days before it, based on sperm surviving up to five days in good cervical mucus and the egg surviving about 24 hours. The two to three days before ovulation are the most fertile.
- NHS, Periods. Cycles happen around every 28 days for most women, and commonly anywhere from every 21 to every 35 days.
Placing ovulation 14 days before your next period is an estimate that assumes a regular cycle. If your cycles vary, the predicted date moves with them, which is the main limitation of any calendar method.
Calculator version 1.0. Figures and formula last checked 27 July 2026 against the sources above. This tool gives general information and does not replace advice from your own doctor.
Frequently asked questions
When am I most fertile?
The fertile window is the five days before ovulation plus ovulation day. The two days before ovulation are the most likely for conception.
How accurate is an ovulation calculator?
It estimates from an average cycle. Real ovulation shifts with stress, illness and cycle changes, so treat it as a planning guide, not a precise date.
Can I use it to avoid pregnancy?
No. It is not a contraception method. Ovulation is too variable to rely on for preventing pregnancy.
What if my cycles are irregular?
The estimate is less reliable, and tracking signs like basal temperature or ovulation tests helps. Very irregular cycles are worth discussing with a doctor. See perimenopause.